1. The Field of the Invention
The present invention is related generally to thin film optical coatings for producing color. More specifically, the present invention is related to multilayer thin film interference coatings or flakes which exhibit color shifting properties with changes in angle of incident light or in viewing angle
2. The Relevant Technology
Color shifting pigments and colorants have been used in numerous applications, ranging from automobile paints to anti-counterfeiting inks for security documents and currency. Such pigments and colorants exhibit the property of changing color upon variation of the angle of incident light, or as the viewing angle of the observer is shifted. The primary method used to achieve such color shifting colorants is to disperse small flakes, which are typically composed of multiple layers of thin films having particular optical characteristics, throughout a medium such as paint or ink that may then be subsequently applied to the surface of an object.
The color shifting properties of the colorant can be controlled through proper design of the optical coatings or films used to form the flakes. Desired effects can be achieved through the variation of parameters such as thickness of the layers forming the flakes and the index of refraction of each layer. The changes in perceived color which occur for different viewing angles or angles of incident light are a result of a combination of selective absorption of the materials comprising the layers and wavelength dependent interference effects. The absorption characteristics of a material as well as interference phenomena are responsible for the basic color which is observed. The interference effects, which arise from the superposition of the light waves that have undergone multiple reflections and transmissions within the multilayered thin film structure, are responsible for the shifts in perceived color with different angles.
Examples of color shifting thin films are disclosed in U.S. Pat. No. 4,705,356 to Berning et al. In one embodiment therein, a three layer metal(1)-dielectric-metal(2) stack is disclosed in which metal(1) is a relatively thin, highly absorptive material, metal(2) is a highly reflecting, essentially opaque metal, and the dielectric is a low index of refraction material. In an alternative embodiment, an all-dielectric stack is disclosed. These designs, however, require the use of so-called subtractive colorant carrying superstrates which utilize dyes to achieve the desired colors.
Further improvements in the optical characteristics of thin film flakes which may be used in paints and inks such as for anti-counterfeiting applications are disclosed in U.S. Pat. No. 5,135,812 to Phillips et al. According to this patent, a symmetrical optical multilayer film is composed either of transparent all-dielectric stacks, or transparent dielectric and semi-transparent metallic layered stacks with an opaque reflector in the center of an optical stack. In the case of an all-dielectric stack, the optical coating is made of alternating layers of high and low refractive index materials. Suitable materials disclosed are zinc sulfide or titanium dioxide for the high index layers, and magnesium fluoride or silicon dioxide for the low index layers.
In U.S. Pat. No. 5,278,590 to Phillips et al., a symmetric three layer optical interference coating is disclosed which comprises first and second partially transmitting absorber layers which have essentially the same composition and thickness, and a dielectric spacer layer located between the first and second absorber layers. The dielectric layer is composed of a material having a low index of refraction such as magnesium fluoride.
High chroma interference platelets for use in paints, including color shifting and nonshifting single color platelets, are disclosed in U.S. Pat. No. 5,571,624 to Phillips et al. These platelets are formed from a symmetrical multilayer thin film structure in which a first semi-opaque layer such as chromium is formed on a substrate, with a first dielectric layer formed on the first semi-opaque layer. An opaque reflecting metal layer such as aluminum is formed on the first dielectric layer, followed by a second dielectric layer of the same material and thickness as the first dielectric layer. A second semi-opaque layer of the same material and thickness as the first semi-opaque layer is formed on the second dielectric layer. For the color shifting designs, the dielectric materials utilized have an index of refraction less than 2.0. For the nonshifting designs, the dielectric materials are selected to have an index of refraction greater than 2.0.
The prior color shifting interference films and platelets which can be used in pigment compositions such as those discussed above typically include metal reflector and absorber layers. In order to utilize such pigment compositions in the broadest variety of applications, it is desirable to eliminate metals which might be environmentally unstable, or otherwise reactive with a paint, ink, or other organic vehicle. While a wide variety of metals or metal compounds may be utilized to form pigments exhibiting color shifts, it is desirable to provide pigment products which are substantially devoid of metals or metal compounds, which are especially suspect for toxic effects in food contact applications or other applications with a potential for exposure to human tissue.
It is an object of the invention to provide color shifting multilayer interference films that can be formed into interference flakes for use as a pigment or formed into foils.
A further object of the invention is to provide color shifting interference films, foils, and flakes which have substantial shifts in chroma and hue with a varying angle of view.
It is yet another object of the invention to provide color shifting interference flakes which may be easily and economically utilized in colorants such as paints and inks for various applications.
An additional object of the invention is to provide color shifting pigment compositions that are substantially free of metals and metal compounds.
To achieve the forgoing objects and in accordance with the invention as embodied and broadly described herein, a color shifting carbon-containing multilayer interference film is provided which may be used to produce flakes for use in pigment compositions or to produce foils having color shifting properties. The flakes can be interspersed into various pigment media such as paints, inks, or cosmetic preparations, for subsequent application to objects, papers, or people to achieve color variations upon changes in angle of incident light or viewing angle. The foils can be adhered directly to objects to provide color shifting properties.
Three and five layer designs of the interference film include alternating layers of a dielectric material and carbon in various configurations. The dielectric layers are formed to have an optical thickness at a design wavelength that provides a color shift as the angle of incident light or viewing angle changes.
In a method of fabricating a color shifting interference film according to the present invention, the carbon and dielectric layers are formed on a web material by conventional deposition processes to produce a particular film design. The film is then removed from the web material in the form of flakes or as a foil. The flakes can then be dispersed in a pigment medium to produce a color shifting pigment composition such as a color shifting paint, ink, or cosmetic preparation.
Alternatively, a color shifting interference pigment can be made by forming a first layer of a carbon or dielectric material on an upper surface of a web material, and removing the first layer from the web material in order to produce a plurality of flakes. The flakes are then coated with one or more additional layers of carbon or dielectric material that substantially surround the flakes, with the adjacent additional layer being formed of a different material than the first layer.
These and other aspects and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.